Field of the Invention
The present invention relates to drive control of a movable member or conveyance control of an object, and more particularly, to a technology for detecting a moving amount of a movable member or an object.
Description of the Related Art
The implementation of a further high speed is requested for image forming apparatuses such as a printer and a copy machine. On the other hand, it is naturally requested to maintain the printing quality to be the original quality or higher. In order to deal with such contradictory requests, drive control of movable members including conveyance rollers and image carrying rotary bodies such as a photosensitive drum and an intermediate transfer belt and conveyance control of sheets need to be more accurate.
As one of devices for further improving the accuracy of such control, it may be considered that the positions or the speeds of movable members, sheets, and the like are detected at a higher speed with higher accuracy and are fed back to such control systems. For this, it is necessary to devise a technology for detecting the moving amounts of detection targets with higher accuracy in real time.
For the detection of the moving amounts at a high speed with high accuracy, optical detectors are advantageous. As main principles of the operation thereof, the following three types are known. A detector of a first type, like a rotary encoder or the like, determines the position of a detection target based on a change in the amount of light that is caused by the detection target blocking or transmitting light transmitted from a light source toward an optical detector (for example, see JP 2014-159323 A). A detector of a second type repeatedly captures images of a detection target using an imaging device such as a complementary metal-oxide-semiconductor field-effect transistor (CMOS) or a charge coupled device (CCD) and calculates the moving amount of the detection target based on a change in consecutive two images (for example, see JP 2014-052437 A and JP 2015-068809 A). A detector of a third type calculates the speed of a detection target based on a change between the frequencies of emission light emitted toward the detection target and reflection light reflected by the detection target that is caused by a Doppler effect.
Particularly, the detector of the second type can detect the moving amount of a detection target with higher accuracy as the structure of the detection target is finer. Accordingly, the detector of such a type captures an image of shades or speckles caused by fine (for example, about several μm) irregularities of the surface of the detection target by using a light emitting diode (LED) or semiconductor laser as a light source. Between consecutive two images, brightness distributions representing the patterns of the shades or the speckles are correlated, and a peak of a correlation coefficient is displaced according to the movement of the detection target. The detector of the second type measures the amount of the displacement in pixel pitches and calculates the product of a measured value and the magnification of the imaging optical system as the moving amount of the detection target.
The detector of the second type can finely configure the measurement precision of the peak displacement of the correlation coefficient up to a sub pixel size by performing a sub pixel process for brightness distributions measured in pixel pitches (for example, see JP 2015-068809 A). In the “sub pixel process”, brightness of an image output in the pixel pitch is interpolated between pixels adjacent to each other, and accordingly, a brightness change in the sub pixel pitch (for example, 1/10 of the pixel pitch) is estimated.
The moving amount detector of the second type, as described above, calculates the moving amount of a detection target based on a displacement amount of a peak of the correlation coefficient between two consecutive images. This displacement amount can be identified at the pitch of a pixel or a sub pixel of an imaging device, and a lower limit of a detectable moving amount is the product of the size of the pixel or the like and the magnification of an imaging optical system. This means that at least a time for a detection target image to move the size of the pixel or the like within an imaging device is necessary. In other words, in a case where an interval (hereinafter, referred to as a “sampling period”) of the imaging time of an image is shorter than this time, the moving amount cannot be detected based on the two consecutive images. Thus, in a case where the moving amount detector of the second type is used for the drive control of movable members or the conveyance control of sheets in an image forming apparatus, there are the following problems.
The image forming apparatus changes the conveyance speed of a sheet based on the sheet type or the thickness of the sheet. For example, a cardboard is conveyed at a speed lower than the speed of a plain sheet. The main reason for this is that the power consumption of a drive motor of the conveyance rollers and the like is stabilized regardless of a weight of a conveyance target sheet. In a case where the conveyance speed of a sheet is decreased, the transfer timing of a toner image from an image carrying rotary body onto the sheet is delayed, and accordingly, the image forming apparatus decreases also the processing speed of image generation and the like. In other words, together with the moving speed of the movable members contributing to the conveyance of a sheet such as the rotation speed of the conveyance roller, the moving speed of movable members contributing to the process of image generation and the like such as the rotation speed of a photosensitive drum and an intermediate transfer belt is decreased. As a result, a time required for a detection target image to move the size of a pixel or the like within the moving amount detector increases. In a case where this moving time exceeds the sampling period, the moving amount of a detection target cannot be detected based on a change between two consecutive images, and accordingly, a conventional moving amount detector needs to increase the sampling period such that a change appears between two consecutive images. In accordance with this, a time interval at which a subject of the conveyance control of a sheet or the drive control of a movable member receives feedback of the moving amount from this detector increases, and accordingly, such control may be easily delayed with respect to a load change. Accordingly, in a conventional detector, it is difficult to maintain responsiveness of the control to be good regardless of a decrease in the conveyance speed of a sheet by maintaining a feedback interval of the moving amount to be constant.